Signal seeking radio receivers of the type used to monitor public service bands are well known. These radio receivers monitor a plurality of discreet and separate channels sequentially and stop whenever a signal is received on a sampled channel. An example of one such prior art receiver is disclosed in U.S. Pat. No. 3,824,475 to Pflasterer. These receivers suffer from the shortcoming of requiring a separate crystal for each of the channels to which the receiver tunes. For radio receivers having a large number of channels, the cost of the large number of crystals required becomes rather substantial. Moreover, in order to change frequencies from those being used in the receiver, new crystals must be purchased to replace those undesired frequencies with the desired frequencies. Not only are the additional crystals expensive, but in addition the delay in ordering crystals for the desired frequencies can sometimes be substantial. Still further, crystals are occasionally defective, off frequency or inoperative when received from the crystal manufacturer thereby necessitating further delays. Certain crystal frequencies are not stocked and must be made to order, thus delaying useful operation of the receiver by the owner for a considerable time.
Phase locked loop circuits for use in synthesizing local oscillator frequencies have been known for many years. In prior art radio receivers of this type, a digital phase locked loop frequency synthesizer is often comprised of a reference oscillator, a phase detector, loop amplifier and filter, voltage tuned oscillator and digital divider. Such synthesizers are utilized to provide any one of a plurality of discrete frequencies for use in a variety of applications such as in scanning radio receivers which scan all channels from one end of a band to the other. The output terminal of the voltage tuned oscillator is connected to the input terminal of the divider, the output terminal of which is connected to an input terminal of the phase detector. The reference oscillator output terminal is connected to another input terminal of the phase detector. The output signal of the phase detector, which is proportional to the phase difference between the divider output signal and the reference oscillator signal, is fed back through the loop amplifier and filter to the voltage tuned oscillator, thus closing the loop.
With the reference frequency (obtained directly from an oscillator or by division from a stable higher frequency oscillator) less than the desired output frequency of the voltage tuned oscillator, the output frequency of the voltage tuned oscillator is divided by an integer n. The output of the divider is compared with the reference frequency within the phase detector and any phase difference is fed back to the voltage tuned oscillator in the form of a control voltage which corrects the frequency of the oscillator so that the frequency out of the divider becomes equal to the reference frequency and the loop is in lock.
By changing n in integral steps, the output frequency of the synthesizer changes in steps equal to the reference frequency. Thus with a single crystal oscillator, many crystal controlled output frequencies are obtained. Automatic scanning using prior art techniques incorporating phase locked loop frequency synthesizers results in complex, expensive equipment of considerable size and weight. Often times electronic memories are incorporated within the units to retain the information relating to the desired frequencies to be scanned. Such electronic memories have the problem of being volatile and being modified by a loss of power to the receiver or electrical disturbances. Others require batteries to maintain the electronic memories. Once the electronic memory is lost, the receiver must again be reprogrammed to the desired frequency, often by quite cumbersome techniques.
Phase locked loop circuits in radio receivers of the prior art are not well suited to cover a very large frequency range. This is in part because the design of the voltage tuned oscillator normally restrains operation of the circuit to within certain limits which are less than are often desired for a multiple band receiver. The use of a field effect transistor in the voltage tuned oscillator to achieve a low noise oscillator makes the design of an oscillator to cover a broad band of frequencies difficult.